The personal computer has proved to be a boon to the field of data acquisition. A personal computer is a general purpose device which may be configured by software programs and by plug-in peripheral equipment to perform a wide variety of special purpose tasks, including data reduction or computation, data acquisition and control. In the particular area of data acquisition, peripheral devices for performing measurements of physical phenomena and converting such measurements to digital signals conventionally are attached to a personal computer through an expansion bus. Messages are transmitted through the expansion bus to issue commands to instruments and to receive data back in return.
One conventional architecture for data acquisition systems includes a computer which communicates over the IEEE-488 bus with one or more processor controlled measurement instruments. The measurement instruments may include such complex devices as spectrum analyzers, as well as simpler devices, such as volt meters. However, each instrument includes a processor for communicating over the IEEE-488 bus and for controlling the instrument. Each instrument is treated in such a system as an intelligent peripheral which executes its own measurement program as directed through communication over the IEEE-488 with the computer. Processors may include microprocessors, microcontrollers, digital signal processors, etc. As a result, the instruments are expensive, consume relatively large amounts of electrical power, are designed for special measurement functions and are complex to program. Each instrument includes a processor as well as associated supporting logic and clocking circuits. The timing of communications over the bus is independent of the timing of measurements (i.e. the communications and measurement processes are asynchronous), thus increasing the exposure of sensitive instruments to digitally created noise from the bus. Therefore, special precautions must be taken in the design of such instruments to avoid electrical noise problems which could affect the measurements produced.
Another conventional approach to data acquisition systems is to attach a general purpose measurement module to a personal computer through an RS-232 or RS-422 serial port. Although such devices tend to be far less sophisticated than IEEE-488 bus based systems, the approach is substantially similar. Each measurement device has a processor within the device, for controlling the measurement components and processing commands and data to be sent or received through the RS-232 or RS-422 serial port. Measurement timing and communication functions are all controlled by the processor within each measurement device. Communication with the personal computer is achieved through the RS-232 or RS-422 serial port.
When a multiwire cable includes a signal wire carrying a signal that quickly transitions from one voltage level to another (e.g. a digital signal), a spike will inductively couple to all other wires in the cable, at each transition. In the case of a digital signal switching between 0V and 5V, 10 mV to 500 mV spikes may appear on the other signals in the cable. This means that the 10 mV to 500 mV spike is added to the other signals, and results in an error equal to the magnitude of the coupled spike. This is called cable cross-talk. Cable cross-talk may be seen in a conventional data acquisition system by grounding the most sensitive input to such a conventional data acquisition system at the sensor, far from the computer. The output of the grounded sensor should then be digitized, and viewed. The spikes discussed above will be seen in the signal viewed. This noise is added to the sensor signal even when it is not grounded, and therefore determines the maximum possible accuracy of the conventional system that has one multi-signal connector, e.g. a standard DB-25 connector, through which multiple I/O signals pass. In addition to cable cross-talk, noise can originate from ground loops, background electromagnetic fields, or the electromagnetic fields and ground noise injection generated by millions of simultaneously switching transistors inside the computer at MHZ rates. This problem of cable cross-talk affects many conventional systems because they route one or more analog and digital signals in the same cable. It should be noted that analog signals characterized by fast transitions can also cause cross-talk and noise to appear in other signals in a cable carrying such signals.
Many conventional systems suffer from one or more of the problems described above.